DisplayRealType ClassCastException

Hi,

Why is it that these work:

ScalarType type = (ScalarType) ScalarType.getScalarTypeByName("XAxis");
RealType type   = (RealType)   ScalarType.getScalarTypeByName("XAxis");

but 

DisplayRealType type = (DisplayRealType)
ScalarType.getScalarTypeByName("XAxis");

gives:

java.lang.ClassCastException: visad.RealType

Is this a bug or a feature?

Thanks,
Doug

-- 
*----------------------------------------------------------------------*
| Doug Lindholm, Software Engineer          |  E-mail: lind@xxxxxxxx   |
| Research Applications Program             |   Phone: 303-497-8374    |
| National Center for Atmospheric Research  |                          |
| P.O. Box 3000                             |     There's no place     |
| Boulder, Colorado 80307-3000              |        like $HOME        |
*----------------------------------------------------------------------*